A: If you're using digital connections between components, such as HDMI, there's a specific order in which you need to start your components. Basically, you need to first turn on your television, your Audio/Video Receiver (AVR), then the Xbox. This is because each component has to query the component where it's sending data for its capabilities. If you turn the Xbox on before the AVR, the Xbox can't confirm 5.1 is supported by the AVR, so it will default to stereo. So make sure the Xbox 360 is the last component to be turned on. If you use a universal remote, you can change the start order so the Xbox turned on last, as shown below for my Harmony remote.
This isn't an Xbox 360 problem. Rather, it's part of the HDMI EDID requirements, which stipulate how capabilities are detected and the order of startup required.